1. Field of the Invention
The invention relates to microstructured optical fiber.
2. Discussion of the Related Art
Interest in high bandwidth optical fiber communication systems continues to increase, and it is well-known that such high bandwidth systems require very low spreading of light pulses as they propagate through the fiber. In a normal step index fiber, different modes travel through the fiber at different rates, causing the pulses to spread. This spreading hampers high speed transmission, because the spreading leads to overlap between the pulses, with this overlap making it difficult to distinguish the propagating signals. Because of this problem of mode overlap, most long-haul transmission systems use single mode fiber. However, the small core size of single mode fiber, e.g., 8 to 9 xcexcm, makes installation and use of the fiber difficult.
Thus, where possible, e.g., in local area networks and buildings, graded index multimode fibers, which have a larger core, e.g., about 62 xcexcm, that eases installation, are used. Pulse spreading in conventional graded index multimode fiber is reduced by the graded refractive index profile. Specifically, the graded profile affects distinct propagating modes differently, such that the group velocity of the modes are essentially equal. Spreading of propagating pulses is thereby kept to acceptable levels. Unfortunately, fabrication of graded index multimode fiber has some disadvantages. For example, the graded index is most often provided by doping the silica core of the fiber with germanium. Germanium is expensive and requires complex reclamation procedures. Moreover, the bandwidth of the resultant fiber is highly sensitive to the germanium profile, which requires highly controlled processing steps that tend to increase costs.
Thus, improvements in multimode fiber design and fabrication would be desirable.
The invention provides a new type of multimode fiber, capable of being constructed without the need for chemical doping. The fiber contains a core region comprising a first material and one or more axially oriented elements disposed in the first material. (The core region is the region in which light primarily propagates). The axially oriented elements (which typically extend throughout the length of the fiber) are configured to provide a graded effective refractive index profile or to provide more focused interaction with selected modes. The fiber further contains a cladding region surrounding the core region, where the cladding region exhibits a refractive index less than the effective refractive index of the portion of the core immediately adjacent the cladding region, i.e., there exists an index step at the core/cladding interface. This step inhibits loss of optical power that is scattered from the core region. The elements are optionally air-filled capillary holes, and are typically located in a circumferential manner around the center of the fiber.
In a first embodiment, a relatively large number of holes (having diameters smaller than the wavelength of light propagating through the fiber), are used to construct an effective index profile that mimics the profile of a conventional graded index fiber. In a second embodiment, using predicted modal field profiles, axially oriented elements are configured to selectively provide interaction with (i.e., manipulate) particular modes.
The concept of microstructured fibers, e.g., fibers having air holes formed therein, is known. Various groups have studied the properties of fibers that utilize air holes as a cladding structure See, e.g., B. J. Eggleton et al., xe2x80x9cCladding-Mode-Resonances in Air-Silica Microstructure Optical Fibers,xe2x80x9d Journal of Lightwave Technology, Vol. 18, No. 8 (2000); J. C. Knight et al., xe2x80x9cAnomalous Dispersion in Photonic Crystal Fiber,xe2x80x9d IEEE Photonics Technology Letters, Vol. 12, No. 7 (2000); J. Ranka et al., xe2x80x9cVisible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm,xe2x80x9d Optics Letters, Vol. 25, No. 1 (2000); and U.S. Pat. Nos. 5,907,652 and 6,097,870. In such structures, the air holes have typically been relatively large compared to the wavelength of the light, and thus generally provided steps in refractive index from silica to air, with the high index contrast providing some interesting properties.
In contrast to this prior work, the invention reflects the recognition that it is possible to design a microstructure that provides an effective refractive index profile which mimics the profile of a conventional graded index multimode fiber, or which provides selected modal manipulation. This recognition makes it possible to form multimode fiber without the typical problems, e.g., germanium recovery and germanium distribution control. In fact, it is expected that it will be possible to tune the profile for various applications, simply by adjusting the arrangement, e.g., size, location, material, of the axially oriented elements.